Thermoplastic elastomers have been prepared by block polymerization of a diene and styrene to produce polystyrenepolybutadiene-polystyrene or like copolymers which may be designated as S-B-S block polymers. These have the interesting property of being thermoplastic, so that they can be molded or otherwise shaped at a sufficiently high temperature, yet when cooled, will exhibit distinctly rubbery elastomeric properties.
Careful study of the S-B-S thermoplastic elastomers has indicated that when the blocks of the polymer are within a critical range of length, the polystyrene terminal portions of the molecular chains will cluster in domains, with the polydiene central portions extending in a random configuration between nearby polystyrene domains. Vulcanization is not required, since the connection of the ends of the polydiene portions or blocks to the polystyrene domains, plus entanglement of the chains, produces adequate physical cross-linkage.
When an article made from an S-B-S block polymer is heated to the melting temperature of the polystyrene terminal blocks, it can be reshaped and will retain its new shape indefinitely without exhibiting appreciable elastic memory.
For comparision, B-S-B type polymers have also been prepared, but have been described as weak, and have been of no apparent interest to previous workers.